1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Related Art
Information processing system apparatuses using electrophotography have been drastically improved recently. Particularly, optical printers in which information is recorded by means of light have been extremely improved in terms of printing quality and reliability. Such a recording technique, so-called a digital recording technique, is applied not only to printers but also to copiers. It is expected that demands for digital copiers, having various additional image processing functions, may grow more and more. In addition, digital color printers have been also drastically improved along with widespread use and improvement of personal computers. The above-described information processing system apparatuses using electrophotography are hereinafter referred to as “image forming apparatuses”.
A typical image forming apparatus includes an electrophotographic photoreceptor (hereinafter simply referred to as the “photoreceptor”). The photoreceptors are broadly classified into organic photoreceptors and inorganic photoreceptors. The organic photoreceptors are widely used recently because of being easily manufacturable at low cost and having a flexibility in choosing materials such as charge transport materials, charge generation materials, and binder resins.
The organic photoreceptors are classified into single-layer photoreceptors including a photosensitive layer in which a charge transport material (e.g., a hole transport material, an electron transport material) and a charge generation material are dispersed, and multilayer photoreceptors in which a charge generation layer including a charge generation material and a charge transport layer including a charge transport material are overlaid on each other.
Most of practical multilayer photoreceptors are negatively chargeable. In contrast, any positively chargeable multilayer photoreceptor has not yet come into practical use. This is because electron transport materials having good electron transportability, less toxicity, and high compatibility with binder resins have not yet come into practical use.
However, there is a disadvantage that negatively chargeable photoreceptors are much unstably charged by corona discharge compared to positively chargeable photoreceptors, while producing ozone and nitrogen oxides. The ozone and nitrogen oxides produced tend to adhere to the surface of the photoreceptor, resulting in physical and chemical deterioration thereof. For this reason, positively chargeable photoreceptors have an advantage over negatively chargeable photoreceptors in terms of flexibility in use conditions and wide application.
The positively chargeable photoreceptors include single-layer photoreceptors. The single-layer photoreceptors have attracted attention recently for the following reasons: being manufacturable with simple processes; having good optical properties because of including few interfaces between layers; having both positive and negative sensitivity because of including both an electron transport material and a hole transport material; and being evenly chargeable while producing a less amount of ozone.
On the other hand, novel electron transport materials have been developed recently. For example, United States Patent Application Publication No. 2007/0219375 discloses derivatives of a tetracarboxylic acid and a naphthalene carboxylic acid. It is disclosed therein that these compounds have good electron transportability and enhance electrostatic properties of single-layer photoreceptors.
However, these compounds do not have satisfactory stability in electrostatic properties when repeatedly used. In addition, single-layer photoreceptors including these compounds have poor charge stability, and therefore chargeability thereof deteriorates after repeated use. Consequently, abnormal images with background fouling tend to be produced.
Single-layer photoreceptors also have a disadvantage that residual images are easily produced.
In a typical single-layer photoreceptor, charge generation materials are included all over the layer. Therefore, charge generation areas spread all over the layer. When pairs of hole and electron are formed all over the layer, movements of holes and electrons are disturbed due to difference in mobility between hole and electron, structural defect, and recombination. As a result, carriers (i.e., holes and electrons) are retained in a portion which is irradiated with a light beam in an irradiation process. Subsequently, the portion, having a potential difference, is irradiated again in the next charging process. Consequently, image density unevenness is caused in the resultant image.